CSS Flexbox (Flexible Box Layout) is a one-dimensional layout model that lets you arrange elements in a row or column, distributing space and aligning items efficiently. It is activated by setting display: flex or display: inline-flex on a container, which makes all direct children into flex items. Unlike older methods that relied on floats or tables, Flexbox was designed specifically for interface layout.

The order property controls the visual order of a flex item independent of its position in the HTML. All items default to order: 0. Items with a lower value appear first; higher values appear later. Note that this only affects visual order - screen readers and keyboard navigation still follow source order.

Flexbox is a one-dimensional layout system best suited for arranging items in a single row or column - ideal for navigation bars, button groups, and distributing items along one direction. CSS Grid is two-dimensional and handles both rows and columns simultaneously, making it better for overall page layouts and card grids. A common pattern is to use Grid for page structure and Flexbox for components within it.